As well-known in the art, light emitting diodes (LEDs) are the focus of attention in the display field because of their outstanding features such as anti-vibration, high reliability, low power consumption, and so on.
Such LEDs have been used in a broad range of industrial applications of diverse fields, e.g., backlight sources of mobile displays, signposts on highways, airport signs, stock quotation boards, subway guide boards, light emitters installed in vehicles and the like, and also recently applied to traffic signal lamps to reduce energy consumption.
In particular, GaN-based LEDs can easily produce white light by applying yellow phosphor powder to cause the combination of yellow and blue light, the complementary colors. However, they are structurally limited to enhance light extraction efficiency in that laser beams are generated only by spontaneous emission rather than stimulated emission.
Therefore, studies have been made about a resonant cavity LED (RCLED), which is configured by adding a resonator to an LED to improve the straightness and intensity of light, or to reduce the full-width half maximum (FWHM) to several nanometers such that power consumption can be reduced without impairing the brightness. More improved resonant cavities of vertical cavity surface emitting laser (VCSEL) were also attempted for vertical lasing via stimulated emission.
However, the RCLED and the VCSEL have a drawback in that they require a high voltage or much current compared with their outputs, resulting in a great heat loss to occur. In order to remedy the LED problem, there were attempts of using a roughly patterned substrate (PS) LED or developing an LED with a roughened surface (RS), but neither of them could actually solve the high voltage problem.
To resolve the deficiency in the existing RCLED and VCSEL, many researches are actively in process to develop a 3D VCSEL type photonic quantum ring (PQR) laser operating with lower power consumption. The PQR laser, unlike the VCSEL laser, does not require a high current and is highly resistant to heat, whereby photo-conversion and thermal characteristics are remarkably improved, compared with any other LEDs.